Styrene-butadiene polymer dispersions are useful in the production of several products, including vehicle tires, carpet backing, adhesives, foams, paper coatings and asphalt emulsions. There are two common methods for producing styrene-butadiene copolymer dispersions: a low temperature method (i.e., cold polymerization) and a high temperature method (i.e., hot polymerization). The low temperature method of producing styrene-butadiene copolymer dispersions involves polymerizing styrene and butadiene monomers at temperatures typically between 5° C. and 25° C. in the presence of a surfactant and in the absence of a carboxylated acid comonomer to produce a “cold” styrene butadiene rubber (SBR) copolymer. The low temperature method can be used to make high molecular weight polymers without introducing excess crosslinking. Unlike the low temperature method, the high temperature method for producing styrene-butadiene copolymer dispersions involves polymerizing styrene and butadiene monomers at temperatures in excess of 40° C., and generally in the range of 50-95° C., in the presence of a surfactant and a carboxylated acid monomer.
The low temperature method of producing styrene-butadiene copolymer dispersions has generally been used for producing styrene-butadiene polymer dispersions for many of the above-described uses because it can be agglomerated to produce a high solids content dispersion typically in excess of 65% solids and can be crosslinked (i.e., cured) to increase the tensile strength of the SBR without significantly reducing its elongation. The hot polymerization method, on the other hand, generally is believed to be only useful for producing styrene-butadiene copolymer dispersions having a narrow particle size distribution and a solids content below 55% making the dispersions useful for products such as paper coatings where polymer solids greater than 60% are not required and where the presence of carboxylation provides latex particle stability in the high shear environments encountered in the production of such products. Therefore, the hot polymerization method has generally only been desirable for paper coatings and some low solids pressure sensitive adhesive applications.
One issue with low temperature SBR aqueous dispersions is that they generally cannot be used in hot mix asphalt formulations such as those used in road paving and asphalt shingle applications. Hot mix asphalt formulations for road paving must comply with the requirements set forth in the Strategic Highway Research Program (SHRP) including having a desired dynamic shear modulus and stiffness. High molecular weight non-carboxylated SBR aqueous dispersions produce an undesired increase in viscosity of the hot mix asphalt formulation, making it difficult to uniformly apply the formulation to a surface. Low molecular weight non-carboxylated latex polymers can produce lower viscosity hot mix asphalt formulations; however, they do not have the desired SHRP performance properties.